Convergence current regulator



Nov. 24, 1959 E. .1. SMURA CONVERGENCE CURRENT REGULATOR Filed March 26, 1956 Y H G M GU I m 3 0s m v .D Q .0. W w m 3 n v M o A W 8 7 H 4| RS .1 A0 I, C mTR I O I I T- 1| 1 RA I 1 E L LAIII F RU U I E c C R H w/fl 3 f 6 1 6 R N E a T 5 5 MTU H U b REC V 4 4 ALP Tl HEC CS nd \11J FIG. 2

I CONVERGENCE 750K 750K 750K 750K 750K 750K 750K 750K 33Oov 34 33 INVENTOR.

EDWIN J. SMURA ATTORNEY t Stes CONVERGENCE CURRENT REGULATOR Application March 26, 1956, Serial No. 573,991

11 Claims. (Cl. 315-31) The present invention relates to an electronic circuit and more particularly to an electronic circuit for regulating the magnetic focusing of a cathode ray tube.

In certain cathode ray display applications, it is frequently desirable to employ magnetic rather than elecr trostatic focusing. An example of a display system using a magnetic focusing system is the single gun color kinescope shown in Fig. 6, page 1253, Proceedings of the IRE, October 1951. Another specialized type of display in which the present invention is applicable is shown in Figure 1 of the instant application, wherein a character display is formed by passing the beam through a character matrix positioned between the electron emitter and the viewing screen so that the difiused electron beam is formed in the shape of characters found on the matrix. In such a display system, magnetic focusing is employed to compensate for the off-axis deflection of the electron beam caused by the character selection and to reconverge the beam toward the axis, thereby enabling the selected character to be suitably positioned on the character format. In the particular embodiment illustrated in Fig. 1 and described hereinafter, a convergence coil is positioned between a character matrix and an electrostatic deflection system and functions to align the beam with the electrostatic deflection system.

In order to obtain the above described characteristics of magnetic focusing, a particular ratio between convergence coil current and accelerating potential must be maintained, since both effect the display in a manner to be described hereinafter. For a fixed accelerating potential, a constant current must be applied to the convergence coil.

Accordingly, a primary object of the present invention is to provide an improved convergence current regulator.

Another object of the present invention is to provide an improved constant current generator adapted to produce a constant current irrespective of relatively wide variations in power supply potential or load impedance.

Another and still further object of the present invention is to provide an improved focusing control circuit for a cathode ray tube display wherein the convergence current is maintained substantially constant irrespective of relatively wide variation in the power supply voltage and in the convergence coil resistance.

A further object of the present invention is to provide a constant current generator adapted to generate a constant current by maintaining a constant potential across a temperature-stable resistance in series with the load.

Still another object of the present invention is to provide an improved convergence current regulator adapted to automatically vary the convergence current as a function of the accelerating voltage supply of an associated cathode ray tube.

A further object of the present invention is to provide an improved electronic circuit associated with a cathode ray tube display circuit wherein a predetermined ratio atent is maintained between the accelerating voltage and the corresponding convergence current to provide a uniformly focused display of characters on the cathode ray tube.

Another object of the present invention is to provide an improved magnetic lens for controlling the magnetic focusing of a cathode ray tube wherein the characters generated by the beam passing through a character matrix are displayed in substantial alignment.

Other objects of the invention will be pointed out in the following description and claims and illustrated in the accompanying drawings, which disclose, by way of example, the principle of the invention and the best mode, which has been contemplated, of applying that principle. i

In the drawings:

Fig. 1 illustrates in simplified form a cathode ray tube display system which identifies a typical environment for the subject invention.

Fig. 2 illustrates in schematic form the convergence current regulator shown as block 8 in Figure 1.

Before describing the operation of the present apparatus, the theory relating to the focusing action of an axially symmetrical magnetic field produced by current flow through the focus coils will be noted to describe the relationship between convergence current, accelerating voltage and electron beam rotation. The present description relates to an electron optical system with axially symmetric field producing elements. Whenever there exists a region in which there is a varying magnetic or electrostatic field having axial symmetry, this region will have properties analogous to those of an optical lens system. A magnetic convergence coil of a cathode ray tube may be considered such a region in which the index of refraction varies continuously both axially and radially. Such a coil may be considered a magnetic lens for beam convergence. While the action of a magnetic lens from a theoretical standpoint is complicated, the convergence coil may be considered a short lens, the focal length of which can be approximated by the formula wherein j=focal length e: electronic charge m=electronic mass E accelerating voltage H =axial component of the magnetic field produced by the coil formula.

e fH,dz

From this formula, it is evident that the angle of rotation of the electron beam is directly proportional to the flux or current through the convergence coil and inversely proportional to the square root of the accelerating voltage. This rotation isproduced because the force on the electron passing through a magnetic field is perpendicular to its velocity and to the magnetic field, and the electrons tend to follow a helical path. While this rotation is of no particular significance in cathode ray tubes having a point image, this rotation is extremely important in certain applications such as the system illustrated in Figure l.

Another consideration complicating the problem of providing a constant focusing action is the temperature variation in the convergence coil which may result from heating due to thefocus current, to the remainder of the equipment, and to ambient temperature changes.

From the above considerations, it is apparent that with constant accelerating potential, the rotation of the beamin the cathode ray tube will vary directly as the convergence current. In a display system of the type hereinafter described, a constant convergence current is required to produce a uniform beam rotation so that the selected characters on the matrix will be displayed in a uniform vertical and horizontal axis. Any constant rotation of the beam between the character matrix and the cathode ray tube screen can be compensated for by angularly displacing the character matrix a corresponding amount in the opposite direction of the rotation produced by'the convergence coil.

In the-preferred embodiment herein described, current regulation is effected by a circuit which samples the magnitude of the accelerating potential and regulates the current as a function thereof. The subject invention maintains a constant ratio between accelerating-potential and convergence current by automatically varying the focus coil current to compensate for variations in the accelerating voltage of the C.R.T. tube.

7 Referring now to Figure 1, there is illustrated in simplified form a cathode ray display system which func- -tions as an environment for the subject invention. A

cathode ray tube utilizes an electron gun I to provide an 'electron'beam which is suitably diffused by a defocus voltageapplied to the first anode 2. A character selection circuit 3 applies a suitable potential to deflection plates 4a, 4b, 5a and 5b to'defiect thebeam through the selected character in character forming matrix 6, a matrix containing all the characters required for a particular display. An electromagnetic convergence coil 7 provides an axial magnetic field to maintain the shape of the beam as it emerges from the character matrix and simultaneously rotates the beam by an amount depending on the current applied to the coil and aligns the beam with the electrostatic deflection system. The convergence coil has a pair of associated trim coils which may be employed to vary the focusing action of the coilif deof the cathode ray tube by application of a suitable potential. Thefields generated by deflection coils 13 and 14 are transverse as compared to the axial field gener ated by convergence coil 7. The diverging funnel shaped section of the CRT. envelope includes a spiraled resistive ring having a constant resistance per unit length Wound around the inside of the envelope which is connected through a sealed-in conductor 16 to the high voltage supply 9. The conductive coating forming this ring functions as theaccelerating anode, and the final electron accelerating potential is applied between this accelerating anode and the electron gun 1. The character matrix'6 may be angularly displaced by a corresponding 4 amount to compensate for the rotation produced by the convergence coil.

To produce a uniform alignment and correct focus of characters displayed on the cathode ray tube, any variation in the accelerating voltage must be compensated for by a corresponding change in the convergence current produced by convergence'current regulator 8 and thus maintain a substantially constant ratio of accelerating voltage to convergence current;

In view of the previously enumerated possible variations of circuit parameters, the need for a circuit to produce a regulated focus current is readily apparent. The present invention comprises a circuit adapted to produce a constant current assuming a constant accelerating potential, and to automatically vary the current in the convergence coil to compensate for variations in the accelerating voltage supply of the tube.

In addition to correct .focusingof characters, the apparatus inFig. l functions to amplify the selected characters to the size desired for display. .This amplification is a functionof tube structure, and the amplificationfactor will be the ratio of D /D wherein D is the'di'stance from'the center of .the convergence coil to the screen of the CRT. and D is the distance from the character selection matrix to the corresponding point on the convergence coil. The distances D1 and D shown in Fig.1 merely illustrate the points of measurement' and are not to be considered as indicative of the particular ratio employed. e

Referring now to Figure 2, there is illustrated in sche matic form the convergence current regulator shown as block 25in Fig. l. Briefly stated, the present apparatus provides a constant current by maintaining a constant potential'across temperature-stable resistances connected in series with the load. For purposes of the ensuing circuit description, the operation of the subject apparatus under varying low voltage power supply (+250, -150 volts),:-convergence coil resistance and accelerating potential (-3300 volts) will be separately described.

Considering first the variation of the +250 volt supply and assuming the other .circuit parameters remain constant, a constant bias will be maintained on control grids 29 and 31 of cascode-connected feedback ampli-' fiers 23 and 25 through a voltage divider network comprising resistors 33 through 40, 4]., 42, 43 and 44. The circuitfrom ground through the .-330(), volt supply through the above described voltage. divider network, conductor 65, resistors 51, 4 and 47 and the 150 volt supply to ground represents a constant current network. In addition to the cascoded amplifiers, the present apparatus includes a voltage regulator tube 21 and a pentode vacuum tube regulator 27.

Constant current regulating action is effected in the above instance in the following manner. Variation in the low voltage-power supply (+250 volts) causes the plate current-of' the vacuum tube regulator 27 to vary. When :to'rs-4 4,' 43 and 48 to control grid 29 of feedback amplifier 23. Thusthe voltage at the plate 69 of feedback amplifier-25, which is connected through resistor 70 to control grid' 71 ofthe series regulator 27, varies in accordance with'the variation'inthe output of pentode regulator 27, thereby producing a corresponding change in bias at controlgrid 7.1 of regulator 27. Any change in the plate current of vacuum tube regulator 27 produces a corresponding reaction through the above described circuit ari'angement to maintain the convergence current substantially constant.

Assuming an increase in plate current flow resulting from variation of the low voltage supply potential or the resistance of convergence coil 45, the resulting increased potential at terminal 63 will be applied to control grid 31 of feedback amplifier 25. Due to the high impedance of the circuit from terminal 66 to the --3300 volt supply, which represents a constant current generator, the potential at terminal 46 will likewise increase. The resulting increased current flow through cascoded amplifiers 23 and 25 will produce a lower potential at anode 69, which is then applied through resistor 70 to control grid 71 of regulator 27. The plate current flow through vacuum tube regulator 27 is thus decreased by a corresponding amount. The above described action is repetitive andfor all practical purposes may be considered instantaneous, so that the potential at terminal 63 is returned to its original value and the current through convergence coil 45 remains substantially constant.

In this manner, any variation in the +250 volt power supply or in the resistance of the convergence coil is automatically compensated for, and a constant current is maintained. The resistors 49 and 51 are fine and coarse adjustment potentiometers to permit current adjustment through the load, but may be considered constant for a particular application. Potentiometers 55, 57 and 59, 61 are dual connected otentiometers having a single adjustment control which function to control the current through trim coils 58 and 62 respectively to a desired value. The values of these potentiometers may also be considered constant for a particular application. Resistor 73 comprises the plate load for feedback amplifier 25 while resistors 75, 77 and 79 constitute load resistors for the anode, suppressor grid and screen grid respectively of vacuum tube regulator 27 The regulating action effected by variations in the 150 volt supply will be identical to the regulating action resulting from variations in the +250 volt supply with one exception. Since the voltage divider network associated with the -3300 volt supply is returned to ground through the -ISO volt supply, any variation in the 150 volt supply will produce a slight variation in the high voltage supply. Since the magnitude of the -150 volt supply is nominal in relation to the 3300 volt supply, the current variation, if any, resulting from a variation in the high voltage supply will be negligible.

With respect to the variation in convergence coil resistance, a variation in temperature of the convergence coil due to the reasons noted heretofore will produce a corresponding variation in the resistance of the copper. An increase in resistance of the convergence coil due to increased temperature will cause an increased voltage drop across the convergence coil, resulting in a drop in potential across the series regulator tube 27 and a decrease in current flow through regulator tube 27. This decreased current is sensed by the potential variation at terminal 63, and an action takes place to restore the current to its original value through the same circuit as described above for variations in low voltage supply potential. With the current restoration there will be a new grid to cathode potential on regulator tube 27. This action may be considered instantaneous and the current is restored to its initial value.

The above description assumed a constant accelerating voltage supply. Since the angle of rotation of the beam in the cathode ray tube is directly proportional to the convergence current and inversely proportional to the square root of the accelerating voltage, any variation in accelerating voltage must be compensated for by a corresponding variation in convergence current to provide a uniform character rotation. In the embodiment illustrated in Fig. l, the relative percentage eflect of convergence current and accelerating voltage on beam rotation is in a ratio of 2: 1. Therefore a uniform character rotation can be maintained if a 2% variation in accelerating voltage produces a corresponding 1% variation in convergence current. Proportionate variations of this type are caused to occur in the following manner. The voltage developed across resistors 43 and 44 produced by the current from the 3300 volt supply through the high impedance voltage divider network is normally equal to one half the total voltage developed across regulating resistors 47, 49 and 51 by the convergence current. From the preceding description it is apparent that any voltage variation across resistors 43 and 44 resulting from a variation in the accelerating voltage is caused to produce an equivalent change in the voltage developed across the series regulating resistors 47, 49 and 51, due to the regulating action heretofore described. However, since the voltage drop across resistors 43 and 44 is normally only'half the drop developed across resistors 47,

49 and 51, the percentage change of the voltage across resistors 47, 49 and 51 will be one half the percentage change of voltage across resistors 43 and 44.

Assuming that the magnitude of the accelerating voltage supply increases by 2%, for example, the voltage applied to control grid 29 of feedback amplifier 23 becomes more negative, thereby decreasing current flow through feedback amplifier 23 and increasing the potential at the anode of the feedback amplifier 23. Since feedback amplifier 23 is cascode-connected to feedback amplifier 25, an increase in anode potential in amplifier 23 produces a corresponding increase in the cathode potential of feedback amplifier 25, while the potential applied to control grid 31 from the 3300 volt supply becomes more negative, both effects combining to decrease the tube bias. The plate current flow through amplifier 25 decreases, increasing the potential at anode 69, this increase being then applied to control grid 71 of pentode regulator 27. Pentode regulator 27 thereupon conducts more heavily, causing the potential applied through terminal 63, conductor 65 and resistor 67 to control grid 31 of feedback amplifier 25 to become less negative. The above described feedback action and the stabilizing effect of voltage regulator tube 21 tend to return the potential at control grid 29 of feedback amplifier 23 to within the above described variation of its original level. In this manner, an increase in magnitude of 2% in the accelerating voltage supply produces a corresponding 2% increase in the voltage between control grids 29 and 31 across resistors 43 and 44. Since this voltage change also appears at terminal 63, and since the voltage between terminal 63 and the l50 volt supply is twice that developed between the grids of the feedback amplifier' as heretofore described, the current increase through resistors 47, 49 and 51 is 1%, the percentage increase in the current through convergence coil 45 is also 1%. A decrease in the magnitude of the accelerating potential functions in a similar manner to produce a corresponding decrease of current through the convergence coil 45. That is, a 2% decrease in accelerating voltage supply will produce a 1% decrease in convergence current.

From the above description it is apparent that a voltage regulator tube such as tube 21 is required to maintain a constant ratio between accelerating voltage and convergence current. Since the potential across resistors 43 and 44 is E/2, the potential between point 63 and -ISO volt supply being E, a constant voltage must be provided in the circuit. This is more clearly demonstrated if the circuit from terminal 46 through resistors 43 and 44 to terminal 63, through resistors 51, 49 and 47 to the volt terminal, through voltage regulator tube 21 and the cathode to grid of feedback amplifier 23 to point 46 is considered as a separate network. In accordance with Kirchhoifs law, the potential between the 150 volt supply terminal and point 46 must also be E/ 2, and voltage regulator tube 21 is used to provide this fixed potential. The cathode of feedback amplifier 23 is thus maintained at a constant potential to provide afixed'bias. I n 7 "In this'manner the beam rotation producedby'convergence'coil 45 is maintained constant, irrespective at variations in the low voltage supply, the'resistance of the convergence coil, 'or variation in the accelerating voltage supply. .Once the convergence current're'gulator circuit is turned on, a uniform display will befprovided on the screen of the cathode ray tubedespite anyof the aforecited variations.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodimenhitwill be understood'that various omissions and substitutions and changes in the form and details of the device illustrated and in its operation may be made by those 'skilled'inthe art without departing from the spirit of 'theinvention. It is the'intention, therefore, tobe limited only as indicated by the scope of the following claims.

What is claimedis: I V

1..A' circuit adapted to apply a constant current'to a load despite a variation in supply potentials or the re sistance of said load comprising a voltage regulatortube for providing a fixed reference potential,=a plurality of vacuum tubes, each of 'said tubes having at 'leastacathode, an'anode anda control grid, means interconnecting the cathode of a first one of said vacuum tubesrto the anode of said voltage regulator tube, means connecting the anode of said firstvacuum tube to the cathode ofa second of said plurality of vacuum tubes, 'means'connecting the anode of said second vacuum tube to the control 'grid' of a third of said plurality of vacuum tubes, an output circuit associated with the cathode of said third vacuum tube, said output circuit including a focus coil, the resistance of which varies in response to variations in temperature,'means interconnecting said output circuit to the cathode of saidvoltage regulator tube and means interconnecting a point in said output circuit'to the anode of said voltage regulator tube and'cathode of said firstvacuum tube.

2. An electronic circuit for maintaining a constant; current through a focus coil despite variations in anode potential or focus coil resistance comprising a discharge circuit having at least a cathode, an anode and a control'electrode, an input circuit associated with said control electrode, an output circuit including atemperaturestable resistance associated with said cathode, a source of anode potentialassociated with said anode, and means responsive to .the variation in potential in said output circuit resulting fromyariation in said anode potential or the resistance of' said focus coil for compensating for said variation, said means including a plurality of cascode-connected vacuum tubes, the inputs to said cascode-connected vacuum tubes varyinga's functions of the output potential of said discharge device and the output of said cascode-connected vacuum tubes being applied to the input circuit associated with said discharge device. 7

3. A circuit for controlling the current through the focus coil of' a'cathode'ray tube as a function of the accelerating potential of said cathode ray tube comprising a focus coil associated withsaid cathode ray tube, an energizing circuitassociated with said focus coil, said energizingcircuit comprising a discharge device having control and output circuits, said focus coil being positioned' in said output circuit, and means responsive to variations in the accelerating potential of said cathode ray tube for automatically modifying the signal level applied to said control circuit of said discharge device to maintain a substantially constant. ratio between said accelorating potential andthe current applied'to said focus CO1 4. In a cathode ray tube circuit for maintaining a predetermined relationship between the current applied to a convergence coil' and the high voltagesupplyapplied to the accelerating anodeof said cathode ray tube, .thecom bination comprising a discharge circuit having at least.;a control electrode and an output electrode," a circuitinc luding'said. focuscoil and said-high voltage'supply con nectd to said-outputelec'trode and-a circuit interconnectedfbet-ween'a point in said last name-d circuit aridsaid control electrode, said last named circuit-fcomprisingixa cascode amplifier circuit, the output of said. amplifier beingiconnectedto said control electrode, and function-' ing -under-a varying high'voltage supply :to provide .a variation of the signal applied to said control electrode of said-discharge circuit.

5. A current regulator circuit adapted to maintain a predetermined relationship between the convergence current-and the accelerating voltage of a cathode ray tube despite variations in line voltage, convergence coil resistance or accelerating voltage comprising a discharge device, an input and output circuit associated with said discharge-device, said output circuit including a convergence coil and a plurality of temperature-stable resistancesconnected to the line reference potential, a feedback circuit interconnected between said output and input circuits of said discharge device, a voltage divider network interconnected between said accelerating voltage and said-line reference potential, a gas diode voltage regulator interconnected between said line reference potential and said feedback network for maintaining a constant' potential at. a point in said feedback network, said current regulator circuit functioning to automatically restore' said convergence current to its original value in case of variations in line voltage or convergence coil resistance and to vary said convergence current to maintain said-predetermined relationship between convergence current. and accelerating voltage in the case of variations in 'said' accelerating voltage.

6. An apparatus of the type claimed in claim 5 wherein said feedback circuit includes a pair of cascode-connected amplifiers and wherein the bias for said amplifiers is provided by said voltage divider network.

7. An electronic circuit adapted to maintain predetermined relationship between focus current and accelerating voltage in a cathode ray tube circuit employing e1ectromagnetic focusing comprising a vacuum tube regulator having at least a cathode, an anode and a control grid, an output circuit including a focus coil and a reference terminal connected to said cathode and an input circuit connected to said control grid, said input circuit including a plurality of amplifier circuits associated with said accelerating voltage and adapted to vary the input signal applied to the control grid of said vacuum tube regulator as a function of said accelerating voltage.

8. A convergence current regulator adapted to provide a constant current to the focus coil of a cathode ray tube despite variations in line voltage or focus coil resistance and control the current through said focus coil in accordance with variations in the high voltage supply of said cathode ray tube comprising in combination, a gas filled voltage regulator tube, means connecting the line voltage reference potential to the cathode of said voltage regulator tube, first and second vacuum tubes having at least a cathode, an anode and a control grid respectively, the cathode of said first vacuum tube being connected to the anode of said voltage regulator tube and the anode of said first vacuum tube being connected to the cathode of said second vacuum tube, a voltage divider network connected between said high voltage sup ply and the line voltage reference potential, said voltage divider network providing grid bias to said first and second' vacuum' tubes, a vacuum tube regulator having at least a cathode, an anode and a control grid, means conmeeting said control grid of said vacuum tube regulator tothe anode of said second vacuum'tube, anoutput cire cuit connected to the cathode of said vacuum tube regulator, said'output circuit including the focus coil of said cathode'ray'tube, means responsive to a'potential variation in the output circuit of said vacuum tube regulator resulting from a current variation to vary the bias of said first and second vacuum tubes and thereby vary the signal applied to said control grid of said vacuum tube regulator whereby the current output from said vacuum tube regulator is restored to its initial value.

9. A circuit adapted to maintain a substantially constant flux through the focus coil of a cathode ray tube despite variation of line potential or focus coil resistance comprising a discharge device having a control electrode and an output electrode, an output circuit including a focus coil and a point of reference potential connected to said output electrode, an amplifier circuit comprising first and second vacuum tubes connected in cascode manner, a voltage regulator tube for maintaining a constant potential at the cathode of said vacuum tube, means connecting the output of said amplifier circuit to said control electrode and means for varying the input signal to said amplifier circuit in accordance with any variation in potential at said reference point whereby the output variation from said discharge device is compensated for by a variation in the signal applied to said control electrode of said discharge device.

10. A constant current circuit for providing and maintaining a constant current through an inductive load irrespective of the impedance variation of said load comprising a normally conducting discharge device having an output and an input circuit, means for maintaining a substantially constant bias on said discharge device, and a feedback network connected between said input circuits of said discharge device, said feedback network including said inductive load and further including a voltage regulator.

11. An apparatus of the type claimed in claim 10 wherein said feedback network includes a plurality of cascode connected amplifiers.

References Cited in the file of this patent UNITED STATES PATENTS 

